Locomotives and other large transit vehicles are typically powered by electric traction motors coupled to drive one or more axles of the vehicle. Locomotives typically have at least four axle wheel sets, with each axle-wheel set being connected via suitable gearing to the shaft of a separate electric motor commonly referred to as a traction motor. In a motoring operation, the traction motors are supplied with electric current from a controllable source of electric power, such as an engine driven traction alternator and apply torque to the vehicle wheels which exert tangential force, or tractive effort, on the surface on which the vehicle is traveling, such as the parallel rail of a railway in the case of a locomotive, thereby propelling the vehicle. In an electrical braking mode of operation, the motors serve as axle driven electrical generators such that torque is applied to their shafts by their respectively associated axle-wheel sets, which then exert braking effort on the surface, thereby retarding or slowing the vehicle's progress.
It is important to monitor the rotational speed of the axle wheel sets to limit undesirable operating conditions, such as excessive wheel creep. In some locomotives, the locomotive speed or tangential wheel speed is calculated from measured motor shaft revolutions per minute (RPM) values based on the diameter of the associated wheel. Typically, a speed sensor or revolution counter is coupled to sense the rotational speed of an output shaft of each drive motor. These RPM signals are converted to wheel rotational speed based on a known gear ratio of the mechanical coupling between the motor shaft and wheel axle. Rotational speed is then converted to vehicle linear speed based upon an assumed diameter of each driven wheel. Outputs of the speed sensors may be used to control an adhesion condition of the vehicle, such as creep of one or more wheels.
FIG. 1 shows a block diagram 10 illustrative of a prior art method for controlling a wheel speed of a direct current (DC) motor powered locomotive. The method illustrated by block diagram 10 includes generating a motor voltage control signal 12 based on speed sensor signals 14a . . . 14c provided, for example, by respective wheel speed sensors. In particular, the speed sensor signals 14a . . . 14c are used to generate a reference speed signal 20 that is indicative of an actual speed of the locomotive in block 16 and to generate a maximum speed signal 22 in block 18. The reference speed signal 20 may be subtracted from the maximum speed signal 22 to generate a creep offset signal 24 that is subtracted from a creep limit signal 26 to generate a creep error signal 28 provided to creep control block 30. The above steps assume the locomotive is being operated in a motoring condition. During a braking operation of the locomotive, suitable changes, such as using a minimum speed instead of maximum speed, a polarity of creep, etc., may need to be made in the above steps as would be understood by one skilled in the art. Creep control block 30 generates a creep control signal 32 that may be compared, in block 40, to a reference current signal 34, a voltage limit signal 36, and a reference horsepower limit signal 36 to determine a minimum among the signals 32, 34, 36, 38. A minimum of the signals 32, 34, 36, 38 may then be used to provide a motor voltage signal 12 to control a voltage applied to a traction motor of the locomotive. One problem with speed sensors is that they operate in an extreme environment and are prone to failure. Even one speed sensor failure on a four or six axle locomotive may require limiting, or de-rating, motoring and/or braking of the all axles of the locomotive until the sensor is repaired.